FIGURE 8.15 Rate of S(IV) oxidation by O, in the aqueous phase under conditions typical of a Los Angeles atmosphere, with S02 = 20 ppb and Os = 50 ppb (adapted from Hoffmann, 1986).

Since the ozone oxidation is important at higher pH values, this reaction is particularly important in sea salt particles generated from seawater, which has a pH of ~8. Thus S02 can be taken up into sea salt particles and oxidized by ozone to form sulfate. Because of the presence of buffering agents such as carbonates in the particles, this oxidation continues until the buffering agents are exhausted, at which point the pH falls and the rate of the ozone oxidation decreases; as discussed shortly, reactive chlorine and bromine species may also participate in S(IV) oxidation in sea salt particles (e.g., see Clarke and Radojevic, 1983; Clarke and Williams, 1983; Miller et al., 1987; Chameides and Stelson, 1992a, 1992b; Sievering et al., 1991, 1994, 1995; Clegg and Toumi, 1997; Keene et al., 1998).

e. Oxidation by H202 and Organic Peroxides

Hydrogen peroxide has been shown to oxidize S(IV) relatively rapidly in solution. Furthermore, because it is a highly soluble compound, even gas-phase concentrations in the low-ppb range, which are common, lead to significant concentrations in the liquid phase (e.g., Gunz and Hoffmann, 1990). For example, using the Henry's law constant for H202 of f X 105 M atm"1, a 1-ppb gas-phase concentration would produce at equilibrium at 25°C an aqueous-phase concentration of f X 10~4 mol 1. This is approximately six orders of magnitude greater than the solution-phase concentrations of 03 expected under ambient conditions! In addition to uptake from the gas phase, H202 can be formed in solution through the photooxidation of organics (e.g., Faust et al., 1997; Anastasio et al., 1997). This relatively large concentration anticipated in the solution phase is a significant factor in making H202 responsible for a major part of the oxidation of S(IV) in the aqueous phase (see later).

A second factor in favor of the importance of H202 in the S(IV) oxidation is the pH dependence of the rate coefficient. Figure 8.16 shows the results of some rate

• Martin and damschen x Penkett a Hoffmann ■ Mader

• Martin and damschen x Penkett a Hoffmann ■ Mader

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